1. Field of the Invention
The present invention relates to a frame structure of a vehicle body and particularly to a front frame structure of a cab over type vehicle with a chassis frame.
2. Prior Art
Generally, in cab over type vehicles, it is difficult to secure an enough space to absorb impact energy in an event of a head-on collision and therefore it is necessary to absorb impact energy with a far smaller crash stroke than in a bonnet type vehicle. It is well known that a chassis frame structure is effective for this purpose because left and right lengthwise side members of the chassis frame absorb impact energy by collapsing of these side members longitudinally.
However, in the frame structure comprising side members and a front bumper directly connected with the front ends thereof from behind, in the event of the head-on collision, since the impact shock is transferred to the side members directly and momentally, sensing of an impact sensor for an air bag system tends to be excessively sensitive regardless of the magnitude of impact. Because of this, it is very difficult to determine a no-firing zone in which the air bag does not operate when a small or medium collision occurs.
In order to solve this problem, Japanese Unexamined Utility Model Application No. Jitsu-Kai-Hei 5-012361 discloses a technique in which a plurality of ribs are disposed in the side member in the perpendicular direction therewith and thin wall sections are provided at portions near the root of the ribs so that the side member collapses more easily in the lengthwise direction of the vehicle for reducing the resistance of impact at the initial stage of a collision.
These means of using the collapsible side member of the chassis frame can be applied to the bonnet type vehicle which is capable of securing an enough crush stroke therein but can not be applied to the cab over type vehicle which must absorb impact energy within a very small crash stroke. Consequently, in the cab over type vehicle having a chassis frame, generally the front bumper overhanged ahead at a given distance from the front end of the side member (front cross member) and the front part of the cabin absorb impact energy by the deformation of the both when a small or medium impact is subjected.
However, this overhang type structure includes miscellaneous disadvantages such as a lack of resistance at the initial stage of a collision, a delay of rise of impact load, an upsizing of the bumper, an unsteady absorption of impact energy and an obscure sensing of the impact sensor.